The present invention relates to magnet systems, and more particularly to driver coils and cooling techniques for use in resistive electromagnets. The present invention finds particular application in the field of magnetic resonance imaging (MRI).
MRI is widely used for imaging anatomical and other structures. A commonly used type of magnet for MRI systems is the superconducting type, which is capable of generating field strengths in the range of 0.5-2 Tesla (T). Superconducting magnets are, however, relatively expensive to manufacture and operate.
At lower fields, for example in the range of 0.1 to 0.3 T, resistive magnets are commonly used to produce magnetic fields for MRI applications. Resistive magnets typically include one more magnet driver coils operating in cooperation with a suitable magnet structure, the magnetic field strength being generally proportional to the current through the coils. A limitation on magnet performance, however, is the heat generated in the resistance of the driver coils. As a result, cooling systems have been used to remove this excess heat.
To provide the necessary cooling, annular cooling flanges or discs fabricated from a material such as aluminum have been placed in thermal contact with the magnet coils. Channels having a size and depth sufficient to accommodate and retain copper tubing having a round cross section have been included on one side of the cooling flange, which has had a thickness greater than that of the tubing. The channels and tubing are arranged in a bifilar wound pattern so that the inlet and outlet of the tubing are both accessible from the outer radius of the cooling flange. The cooling flanges have then been placed against the driver coils, with the tubing preferably on the side of the flange facing away from the magnet coil. In operation, a coolant such as water has been caused to flow through the tubing.
In practice, however, neither the surface of the driver coil nor surface of the cooling flange are perfectly flat. The resultant gaps degrade the thermal conductivity between the coil and flange. There have also been radial gaps between the channels and hence the turns of tubing, also reducing thermal efficiency.
In magnet systems having two driver coils on each pole of the magnet it has been necessary to use three cooling flanges. The structure associated with each pole has thus included a first cooling flange, a driver coil having its first side electrically insulated from but in thermal contact with the first cooling flange, a second cooling flange located between the second side of the first driver coil and first side of a second driver coil (the second cooling flange being in thermal contact with but electrically insulated from the driver coils), the second driver coil, and a third cooling flange electrically insulated from but in thermal contact with the second side of the second driver coil.
Driver coils have also included many turns of a conductor such as aluminum arranged in a generally planar, disc-shaped coil. The conductor has had a rectangular cross-section, with the conductor wound or coiled from an inner to an outer radius in the form of an annulus or disc. To insulate between the multiple conductor turns, an anodized aluminum conductor has been used. A disadvantage of anodized aluminum, however, is its cost. A further disadvantage is that defects in the anodization may result in short circuits between coil turns, with a corresponding deleterious effect on magnet performance.
While prior art magnet coils have proven satisfactory, there remains room for improvement. To improve magnet performance, it is also desirable to improve the efficiency of the cooling system, not only to reduce the absolute temperature of the magnet coils, but also to minimize temperature gradients within the coils themselves. It is also desirable to reduce the cost and complexity of the cooling system. In addition, a technique for insulating the turns of the magnet coil which avoids the disadvantages of the anodized approach but which does not degrade the performance of the cooling system is needed.